This invention relates to a semiconductor device, and more particularly to a gate turn-off type thyristor (which is hereinafter referred to as "GTO" type thyristor) having a turn-off function.
A conventional GTO type thyristor is generally constructed as shown in, for example, FIG. 1. Namely, the conventional GTO type thyristor comprises a semiconductor material body composed of four successively continguous layers - a first P type semiconductor layer 11, a first N type semiconductor layer 12 formed thereon, a second P type semiconductor layer 13 formed thereon, and a second N type semiconductor layer 14 formed thereon, said second N type layer 14 being formed into a plurality of independent N type layer portions 14a, 14b, 14c, . . . , and is constructed such that on the respective surfaces of the first P type layer 11, second P type layer 13, and second N type layer portions 14a, 14b, 14c, . . . a first electrode 15, second electrode 16 and third electrodes 17a, 17b, 17c, . . . are provided, respectively; and a first metallic member 18 constituting a first package electrode is provided on the first electrode 15 and a second metallic member 19 constituting a second package electrode is provided on the third electrodes 17a, 17b, 17c, . . . In the GTO type thyristor having the above-mentioned construction, the first P type layer 11 is used as an anode region, the first N type layer 12 is used as a base region, the second P type layer 13 is used as a gate region, and the second N type layer 14 is used as a cathode region. Note that in this specification the GTO type thyristor is defined to mean a thyristor element which is capable of being rendered nonconductive, by applying a reverse bias current signal to the gate of the thyristor element kept in a conductive state.
The above-mentioned conventional GTO type thyristor is generally manufactured as follows.
First, as shown in FIG. 2A, an N type silicon substrate 12 is prepared. Then, by diffusing a trivalent impulity, for example, boron (B) into the substrate 12 first and second P type layer 11, 13 are formed therein as shon in FIG. 2B. Subsequently, by diffusing a pentavalent impurity, for example phosphorus (P) into the P type layer 13, a second N type layer 14 is formed therein as shown in FIG. 2C. Further, by mesaetching the second N type layer 14 at prescribed portions thereof, the layer 14 is divided into a plurality of N type layer portions 14a, 14b, 14c, . . . as shown in FIG. 2D. Next, by vacuum-depositing aluminum on the semiconductor material body constructed into the above-mentioned four-layer structure, a first aluminum electrode 15, second aluminum electrode 16, and third aluminium electrodes 17a, 17b, 17c, . . . are formed, as shown in FIG. 2E, on the first P type layer 11, second P type layer 13, and second N type layer portions 14a, 14b, 14c, . . . , respectively. Thereafter, as shown in FIG. 2F, a first metallic member 19 constituting a first package electrode is disposed commonly on the third aluminium electrodes 17a, 17b, 17c, . . ., while a second metallic member 18 constituting a second package electrode is disposed on the first aluminium layer 15. The GTO type thyristor shown in FIG. 1 is formed by the foregoing manufacturing process.
Generally, it is impossible from the standpoint of manufacturing technique that the semiconductor layers constituting the semiconductor material body each have a uniform impurity concentration over the entire region of a given plane intersecting the thickness direction of each layer at right angles thereto. Further, it is also impossible, due to the incomplete crystalline structure of the semiconductor material body used or due to the lattice defects in the crystal thereof, that the lifetime of minority carriers injected into ech semiconductor layer becomes equal over the entire region of a given plane intersecting the thickness direction of each layer at right angles thereof. Further, generally, in order to permit the turn-off function to be reliably achieved and also to permit a large current to flow in the GTO type thyristor, this thyristor has its cathode layer constituted by a plurality of mutually independent cathode layer portions as shown in FIG. 1. These cathode layer portions are usually formed by mesa-etching the cathode layer formed on the gate layer. In this case, however, it is impossible from the technical point of view of form such plurality of cathode layers into the same configuration and dimension.
For the above-mentioned reasons, the conventional GTO type thyristor has the following drawbacks due to the occurrence of variation in the amount of current flowing in the thyristor units (which are defined to mean, in FIG. 1, the one comprised of the anode region 11, base region 12, gate region 13 and cathode region 14a, the one comprised of the anode region 11, base region 12, gate region 13 and cathode region 14b, and the one comprised of the anode region 11, base region 12, gate region 13 and cathode region 14c, respectively).
(1) The total amount of current capable of being passed through the thyristor element as a whole can not be increased.
(2) Where the above-mentioned current variation is extremely wide, a large current concentratedly flows in a particular thyristor unit, which causes a damage thereto, which results in a damage or breakage of the thyristor as a whole.
(3) The turn-off times for the thyristor units differ from each other.